This invention relates to plants of cotton (Gossypium hirsutum L) having new and improved characteristics, and in particular to cotton plants which have the property of self-defoliation. The invention also relates to the gene or genes which determines the self-defoliating property.
The production of cotton is a major industry in many-countries, including Australia. All cotton fibre is produced from plants of the genus Gossypium. The most commonly grown cotton plants are varieties of Gossypium hirsutum (American Upland cotton), which produces fibres of medium staple length, and these are grown in the United States, Australia, Pakistan and other countries where extensive irrigation is available. Egyptian cotton, which has a finer, longer fibre, is produced from Gossypium barbadense, grown extensively in Egypt and Sudan. Gossypium herbaceum and Gossypium arboreum are grown in unirrigated areas of India, Pakistan and other Asian countries, and produce coarser, shorter fibres.
The purity of commercial seed stocks is carefully controlled to avoid problems resulting from crossing between varieties or seed mixing.
However, the cultivation of cotton plants traditionally has required high-intensity agricultural practices, including heavy irrigation, and the application of a number of pesticides. Cotton plants are prone to disease and to infestation by a variety of insect pests, such as Heliothis caterpillar, and the various species of cotton boll-worms, and hitherto control of these pests has required intensive use of chemical insecticides. Furthermore, because of the requirements of mechanical harvesting, defoliants are applied just before harvest in order to remove the leaves from the plant so as to render the cotton bolls easily accessible to the harvesting machinery.
Consequently the cotton-growing industry has been the cause of considerable environmental pollution, and the industry is under great pressure to reduce release of chemicals into the environment. Integrated pest-management practices are increasingly being used, and cotton plants genetically engineered to be resistant to disease or which express Bacillus thuringiensis toxin, a natural insecticide of bacterial origin, are becoming available to commercial cotton growers. However, hitherto there has been no alternative to the use of chemical defoliants before harvest.
Whilst wild species of perennial cotton such as G. aridum genom D4, G. gossypioides D6 and G. trilobum D are said to lose leaves acquired during the rainy season when the dry season arrives, this self-defoliation to date has not been found in cotton strains grown commercially.
For many years, traditional breeding methods have been used in an endeavour to identify and select strains of cotton which have improved resistance to the major insect and fungal pests which attack these plants, or which have other desirable characteristics. In parallel, breeding programs have also been directed to the production of self-coloured cotton, which does not require the use of chemical dyes during textile processing.
Professor Victor Fursov, a member of the Academy of Technological Science of the former Soviet Union, commenced cotton plant development programs in March 1962.
The program between 1962 and 1993 involved the development of a strain of cotton with specific characteristics, bred and tested under commercial conditions. These included strains of cotton in varying colours of green, beige, brown and xe2x80x9csnow whitexe2x80x9d. Surprisingly, the beige and brown self-coloured strains were found to have superior resistance to major insect and fungal pests and had bactericidal properties.
Further development, generation of strains and selection carried out in Australia identified certain strains which have the property of self-defoliation, and which do not need application of chemical defoliation agents.
In one aspect, the invention provides a self-defoliating cotton plant. In one embodiment, the plant has a gene or functional fragment thereof which is activated to effect self-defoliation of the cotton plant.
In a particularly preferred embodiment, there is provided a strain of cotton (Gossypium hirsutum) characterised in that the plants self-defoliate at the stage of boll opening.
The present invention further provides a self-defoliating plant which includes a nucleic acid or functional fragment thereof which is activated to effect self-defoliation of the cotton plant.
In a second aspect, the invention relates to a self-defoliating cotton plant having a DNA fingerprint as shown in FIG. 3. The plant comprises a nucleic acid sequence which determines self-defoliation of cotton (defoliating gene), which gene can be activated by chemical treatment and irradiation. Preferably the nucleic acid sequence comprises the sequence set out in SEQ ID NO:2. More preferably, the gene is activated by treatment with ethylene imine (Aziridine) and ionising radiation.
In a third aspect, the invention provides a method of activating a defoliating gene in cotton, comprising the step of treating the seed of said cotton with ethylene imine and ionizing radiation. Preferably, hybrid seeds produced by crossing of parent cotton plants which have been selected for desired characteristics or traits are treated with 0.1% v/v aqueous ethylene imine for 10 hours, followed by gamma-irradiation of said seeds with 20 kiloroentgens absorbed dose, with a preferred dose of 4 kiloroentgen for 50 seconds. The irradiation may be suitably effected by exposure of the seeds to a Cobalt 60 gamma-ray source, such as MPX-gamma 3.
In a fourth aspect the invention relates to cotton plants which exhibit self-defoliation. The gene for self-defoliation may be activated by chemical and radiation methods, or may be inherited from a parent plant.
Preferably the cotton fibres are of a colour selected from the group consisting of beige, snow-white, brown and green. Also preferably the cotton plants are resistant to one or more diseases caused by Thielaviopsis babicola, Fusarium vasinfectum and/or Bemisia tabaci. 
In a particularly preferred embodiment, the plant is of a variety selected from the group consisting of Rainbow 34, Rainbow 39, Rainbow 38 and Rainbow 37, as herein described. It will be clearly understood that these varieties have colours of the cotton fibres as the principal characteristic differentiating between them.
The whole plants, seeds, and other reproductive material derived from the plants, including cuttings and protoplasts, all form part of the invention. In addition, products derived from the cotton plants, including cotton fibres and textiles produced therefrom, also form part of the invention.
In a separate aspect, the invention provides cotton plants which have been transformed with the self-defoliation gene of the invention. In particular, genetically-engineered strains of cotton and methods for their production are known. There are a number of patents and literature publications by workers from Agracetus and Monsanto describing methods for transformation of cotton, and transgenic cotton plants expressing exogenous proteins such as Bacillus thuringiensis crystal protein. Such transgenic cotton plants have been widely field tested, and some strains are in commercial production.
For the purposes of this specification, the term xe2x80x9cself-defoliationxe2x80x9d is to be understood to mean the self-shedding of foliage and/or leaves from the lower sections of the plant to the higher points, between the period of the growth cycle from 110 days to 135 days, at which time watering can delay the cycle.
Full boll opening occurs at approximately 110 to 135 days.
The terms xe2x80x9cactivatedxe2x80x9d and xe2x80x9cactivatingxe2x80x9d are to be understood to mean the conversion of the dormant gene for defoliation to one the expression or expression product of which contributes to self-defoliation of a plant containing such a converted gene. The activation includes unblocking of a dormant gene by mutation or by removal of a blocking agent, or by inhibition of an activity thereof. It also includes the inheritance of a gene that was previously activated in the manner described above.
Throughout the description and claims of this specification, the word xe2x80x9ccomprisexe2x80x9d and variations of the word, such as xe2x80x9ccomprisingxe2x80x9d and xe2x80x9ccomprisesxe2x80x9d, means xe2x80x9cincluding but not limited toxe2x80x9d and is not intended to exclude other additives, components, integers or steps.